Method for detecting a load-related change in thermal capacity of a water-bearing domestic appliance

ABSTRACT

A water-bearing appliance, such as a dishwasher, and a method for detecting the load-related change in thermal capacity of the water-bearing domestic appliance, in order to optimize the drying process. In an exemplary embodiment, the method includes detecting a temperature gradient during the cooling of the items to be cleaned.

The invention relates to a method as claimed in the preamble of claim 1.

In water-bearing domestic appliances such as dishwashers, the thermalbehavior of said appliances varies as a function of the amount and typeof wash items, i.e. the items loaded cause a change in thermal capacity,with the result that, for example, the duration of cooling or dryingprocesses is extended or reduced.

WO 2004/047608 A1 discloses a method for detecting the amount of itemsin the washing compartment (tub) of a dishwasher, wherein both motoroperating data of a circulating pump and the so-called heating gradientin the dishwasher are recorded at least in a pre-wash phase and in aheating phase. The actual values captured are compared with the storedsetpoint values and the amount of items in the washing compartment isinferred therefrom. The wash program can then be adapted to suit theamount of items ascertained. This method requires a high degree of open-and closed-loop control complexity, as a large number of curve ormeasurement data scenarios must be stored in a program control unit andcompared with the captured values for the setpoint/actual comparison. Inaddition, the heating power of a dishwasher depends on the locallyavailable electricity supply voltage, with the result that variations inthe locally available supply voltage can falsify the measurement result.

The object of the invention is therefore to provide an improved method.

The object of the invention is achieved by a method for detecting theload-related change in thermal capacity of a water-bearing domesticappliance, in particular a dishwasher, to optimize a drying process.

It is provided according to the invention that a temperature trendduring cooling of the wash items is captured. For example, during themain cleaning cycle, the temperature of the cooling wash liquor, whichis in temperature equilibrium with the wash items, is measured. Becausea large load cools down more slowly than a small one, the capturedtemperature trend of the wash liquor can be used as a measure for theload. It can be captured in a technically simple manner via atemperature sensor, because the sensor can be incorporated in thecirculation path without great technical complexity. Alternatively or inaddition, the temperature trend can be captured on a condensationsurface, e.g. on the inside of a door or on the outside of a water tankused as a container for temporarily storing water and/or wash liquor.Advantageously, in both variants for determining the temperature trend,pairs of temperature values are captured at two different locations inthe appliance. A first value can be determined in an area upstream ofthe wash items, and a second downstream thereof. A temperaturedifference e.g. of the wash liquor can therefore be obtained from valuesbefore and after contact with the wash items. The change in thermalcapacity due to the wash items can be determined from the change in thedifference. Similarly, the correlation between temperature trend andthermal capacity also applies to measuring a temperature on thecondensation surface.

In another embodiment of the invention it is provided that thetemperature trend is captured after mixing of wash liquor with freshwater, i.e. the change in thermal capacity due to the load is determinedcalorimetrically by measuring a mixing temperature from one of the twotemperature values in the event of a change or at least partial changein the wash liquor. This can take place, for example, during thecleaning cycle or an intermediate wash cycle. On completion of a firstcleaning cycle with warm water, it can be wholly or partially pumped offand cold fresh water supplied to the wash tub. The fresh water is heatedby contact with the warm wash items and possibly by mixing with warmwater remaining from the cleaning cycle. Disregarding the temperature ofthe wash items prior to the supply of fresh water, the thermal capacitycan be derived by means of a calorimetric calculation from thetemperature and amount of fresh water supplied, possibly the quantityand temperature of the fresh water remaining from the cleaning cycle,and the mixing temperature. This data can also be obtained in atechnically simple manner—in some cases using means already present,i.e. with a low degree of technical complexity.

Such a procedure would not be convenient. In an advantageous embodimentof the invention, a time dependence of a temperature indicative of thetemperature of the wash items themselves and/or the time dependence of atemperature indicative of the temperature of a condensation surface arecaptured. The time dependence of the temperature of the wash items orcondensation surface is to be understood as meaning the temperaturetrend. The humidity in the wash tub during cooling as part of a dryingprocess is formed on the condensation surface. In particular,determining the temperature on the condensation surface provides acapturing possibility that is both simple and independent of the washliquor and circulating pump or rather its performance data. Theinvention therefore makes use of the recognition that the trend of thewash item temperature, i.e. its change over a particular time period, isdirectly correlated to the thermal capacity and temperature of the washitems. This provides a technically simple calculation method forindirectly determining, or rather estimating within tight limits, theper se difficult to detect size of the thermal capacity.

In the above mentioned embodiments, a fit function describing the timedependence of the temperature during cooling or mixing can be matched tothe time dependence during cooling or mixing, said fit function havingthe thermal capacity of the wash items as a fit parameter. The thermalcapacity of the wash items can also be determined in a simple manner asa measure for the load in this way.

In addition to measuring the temperature trend during a cool-down phaseand/or of a mixing temperature, it can preferably also be provided tocapture the temperature trend during a wash liquor heat-up phase,particularly of re-circulated wash liquor, in order thus to increase theaccuracy by combining these measurements.

The invention also relates to a water-bearing domestic appliance, inparticular a dishwasher, at least having means for detecting theload-related ability to store thermal energy. According to theinvention, the water-bearing domestic appliance has means for measuringa temperature trend during cooling of the wash items. The current loadis determined automatically, i.e. without operator input, therebyconsiderably simplifying the operation of the dishwasher.

According to the invention, the load can be detected indirectly bydetermining the thermal capacity of the wash items. To determine thethermal capacity, the dishwasher can incorporate a temperature sensorfor capturing a temperature indicative of the wash items, and means forevaluating the captured temperature and/or its time dependence. Thetemperature sensor can be disposed in the washing compartment or in thecirculation path and comes into contact with water circulated during acleaning cycle, said water in turn being in heat-exchanging contact withthe wash items. It must therefore be disposed such that it can at leastindirectly capture the temperature of the wash items. A secondtemperature sensor with associated evaluation means for measuring thetemperature of freshly supplied, not yet heated fresh water can also beprovided. In the case of dishwashers of the type incorporating a heatstore, the second temperature sensor can be in heat-exchanging contactwith the heat store. The second temperature sensor and the evaluationmeans enable the thermal capacity of the load to be determined accordingthe method last described above.

The dishwasher can incorporate a control unit which is designed toprocess the data of the temperature sensor(s), i.e. carry out the abovedescribed method or sections thereof and their variants.

The principle of the invention will now be explained in greater detailusing examples and with reference to the accompanying drawings in which:

FIG. 1: shows a temperature trend in the tub of a dishwasher,

FIG. 2: shows a segment of such a temperature trend for different loads,

FIG. 3: shows a schematic sectional view of a first dishwasher, and

FIG. 4: shows a schematic sectional view of another dishwasher.

FIG. 1 shows the known cycles in a dishwasher with residual heat drying.These comprise a pre-wash 2, a heat-up phase 4, a cleaning cycle 6, anintermediate wash cycle 8, a rinse 10, and a drying cycle 12 completingthis sequence of operations. In the pre-wash 2, cold fresh water(approx. 3.4-3.9 l) is supplied and circulated through the wash tub 14(see FIGS. 3 and 4) for a predetermined time of approx. 15 min by acirculating pump 20. A heater 56 (see FIGS. 3 and 4) in the hydrauliccircuit heats up the fresh water of the pre-wash 2 in approx. 13 to 14min to an initial cleaning temperature of approx. 51° C. This also heatsup the wash items 28 in the tub 14. In the subsequent cleaning cycle 6,the heated wash liquor provided with detergent is circulated, therebyessentially cleaning the wash items 28.

Between the cleaning cycle 6 and the intermediate wash cycle 8, the washliquor is pumped out of the tub 14 and clean, cold fresh water issupplied. During the intermediate wash cycle 8, the fresh water iscirculated for a period of approx. 5 min, heating up as it does soprimarily due to contact with or rather heat transfer from the washitems 28 still warm from the cleaning cycle 6 and possibly a heatexchanger 38 (FIG. 4). For the change from the intermediate wash cycle 8to the subsequent rinse 10, the intermediate wash water is pumped out ofthe tub 14 and cold fresh water is re-supplied.

In conventional dishwashers with residual heat drying, the cold freshwater supplied is circulated in the rinse cycle 10 for a predetermined,fixed time of e.g. approximately 15 min during which it is heated to theinitial temperature T_(o) for the final drying cycle 12, e.g. to approx.65° C., using a predetermined, fixed heating power.

FIG. 2 illustrates the trend over time of the characteristictemperature, i.e. the time dependence of the temperature in the tub fordifferent loads during the rinse 10 and drying cycle 12. The middlecurve in FIG. 2 shows the temperature trend in the tub for a definedstandard load B_(standard). The upper and lower curves in FIG. 2represent the temperature trend in the tub for a (compared to thestandard load B_(standard)) higher load B+:=B_(standard)+AB and lowerload B−:=B_(standard)−ΔB respectively. Due to the supply of heat energy,the temperature in the tub 14, and therefore also the temperature of thewash items 28, increases essentially proportionally to the time t duringthe rinse 10. The less than proportional temperature rise shown in FIG.2 is the result of heat transfer losses through the walls of the tub 14and the loading door 16, among other things.

For the standard load B_(standard), the temperature during the heat-upphase in the rinse 10 is adjusted to an initial temperatureT_(0,standard) according to the middle curve in FIG. 2. Immediatelythereafter there commences the residual heat drying cycle 12, i.e. thecomplete evaporation of the water film on the wash items. If a higher orlower load was detected, a correspondingly larger or smaller heat energyinput is required for residual heat drying. Accordingly, the temperatureduring the heat-up phase is set to a higher or lower initial temperatureT_(o)-FAT or T_(o)-AT for the residual heat drying cycle 12.

With the removal of the heating power supplied to the circulated washliquor during the rinse 10, the drying cycle 12 begins. The temperaturein the tub essentially follows a falling exponential function duringwhich a film of moisture present on the wash items 28 evaporates andcondenses on a condensation surface. At a time t₁₂, as a characteristicfeature, a temperature T₁₂ is reached which then changes onlyinsignificantly and marks the attainment of an essentially asymptoticstate. The film of moisture on the wash items 28 is then completelyevaporated and the drying process 12 can be terminated. As the reachingof time t₁₂ is dependent on the load, its detection is criticallyimportant for controlling the drying process in respect of energy inputand time trend.

According to the invention, the time dependence T1(t) of an actualtemperature T1 in the tub during the cool-down phase of the cleaningcycle 6, i.e. the temperature trend over time t, is captured. From thisis obtained the thermal capacity of the load as a measure for the actualload B_(act). The time dependence T1(t) of the temperature during thecool-down phase essentially follows an exponential function in time t

T1(t)≈e ^(−c) ^(tot) ^(·(t-t) ⁰ ⁾  (1)

where C_(tot)=C(B_(act))+C(water) is the total thermal capacity which isunderstood as being the sum of the thermal capacity C(B_(act)) of thecurrent load B_(act) and the thermal capacity C(water) of the circulatedwater. t₀ is the time at which the cool-down phase begins. The thermalcapacity C(water) of the circulated wash liquor depends on the admittedamount of water which is measured when the tub is filled with freshwater. The total thermal capacity C_(tot) is determined by matching afit function to the cool-down curve T1(t) with C_(tot) as the fitparameter. Finally, the change in thermal capacity C(B_(act)) due to thecurrent load B_(act) is calculated by subtracting the measured thermalcapacity C(water) from the thermal capacity C_(tot) derived from thecool-down curve T1(t).

According to an alternative embodiment of the invention for determiningthe change in thermal capacity due to the load, the mixing temperatureobtaining in the intermediate wash cycle 8 is measured. For thispurpose, a function is matched by fitting to the time dependence of thetemperature measured in the intermediate wash cycle 8, and the mixingtemperature obtaining after the supply of the cold fresh water at thestart of the intermediate wash cycle 8 due to temperature equalizationwith the wash items 28 still warm from the cleaning cycle 6 isdetermined as an asymptotic approximation to the temperature-timedependence in the intermediate wash cycle 8 using known mathematicequations or models for calorimetric temperature mixing.

The dishwasher shown in FIG. 3 comprises a tub 14 in which the washitems 28 are placed in a dish rack 30, a loading door 16 attached to thetub 14, a rotary water spray arm 24 pivotally disposed in the tub 14, acirculating pump 20 disposed below a base wall 19 of the tub 14 forcirculating the wash liquor, a feed 22 a connecting the circulating pump20 to the spray arm 24, a drain 22 b in the base wall 19 of the tub 14which is connected to the suction side of the circulating pump 20, aheater 56 on the feed 22 a for heating up the circulated water, a firsttemperature sensor 32 and a second temperature sensor 34, a control unit58 for controlling the cycles and devices of the dishwasher and forreading and evaluating the measurement signals of the temperaturesensors 32, 34, a supply pipe 48 for supplying fresh water, a drain pipe52 for removing used wash liquor, and a heating device 56 on the feed 22a with a control line 56 s to the control unit 58.

The first temperature sensor 32 is disposed in the circulating pump 20and is used to capture the temperature T1 of the water or rather washliquor in the circulation path. However, it can also be disposed inother positions in the circulation path, such as in the feed 22 a, inthe drain 22 b or in a recess in the base wall of the tub 14 near theopening of the drain 22 b. The second temperature sensor 34 is disposedin contact with the inside wall, i.e. the wall of the loading door 16facing the tub 14, and is used for measuring a reference temperature T2indicative of the temperature of a cold surface in the tub 14. It canalso be disposed, for example, in a control panel 18 in the loadingtemperature 16 (sic).

The temperature sensor 32 in the circulating pump 20 captures atemperature trend of the wash liquor over time and forwards the data tothe control device 58. The temperature of the wash liquor is determined,on the one hand, by the output temperature of the fresh water from thedomestic supply pipe. As the fresh water first passes into thecirculating pump 20 before it is pumped further, the sensor 32 is ableto capture its temperature. The heating power subsequently supplied tothe fresh water is likewise known. Largely constant or of at least onlyrelatively slight effect are the energy losses via the line 22 a and thewalls of the tub 14. The control device 58 can therefore determine thetemperature of the wash liquor when it enters the tub 14 before it comesinto contact with the wash items 28.

Also affecting the temperature of the wash liquor is the temperature ofthe wash items 28 on which the wash liquor can be heated or cooled. Whenthe wash liquor is repeatedly circulated e.g. during the heat-up phase 4(cf. FIG. 1), after each discharge from the tub 14 the liquor acquires alower temperature than it had in the feed pipe 22 a because it is cooledon the wash items 28. The control device 58 can infer the degree ofloading of the tub 14 both from the captured temperature differencebetween the wash liquor flowing into and out of the tub 14 and from thechange in said temperature difference over time. For a smaller amount ofwash items 28, a lower thermal capacity is present in the tub 14, whichmeans that the wash liquor is cooled less. The wash items 28 thereforeheat up more quickly, thereby enabling the heat-up phase 4 to beshortened or the power of the heater 56 to be reduced. Conversely, for alarger load it is necessary to extend the heat-up phase 4 or increasethe heating power.

Alternatively or additionally, namely to improve the data set of thecontrol unit 58 for determining the load, a second temperature sensor 34can be mounted in or on the loading door 16. The loading door 16constitutes a relatively cool condensation surface in the residual heatdrying cycle 12. The wash items 28 heated up in the preceding rinse 10evaporate the moisture adhering thereto which forms on the loading door16 as a cool condensation surface. The trend of the temperature of thecondensation surface is also an indication of the degree of loading ofthe tub 14, as a larger amount of wash items 28 can bind acorrespondingly larger amount of moisture on their surface. Thesubsequent condensation delivers more heat to the condensation surfaceof the loading door 16 than a smaller load can.

The second embodiment of the dishwasher shown in FIG. 4 differs from thefirst embodiment shown in FIG. 3 in that it has a water reservoir 38used as a heat store. Identical elements of the first and secondembodiment are denoted by the same reference characters.

The dishwasher shown in FIG. 4 comprises the supply pipe 48 providedwith the controllable valve 50 for filling the heat exchanger 38 withfresh water and a connecting pipe 40 between the heat exchanger 38 andthe circulating pump 20, and also a third temperature sensor 36 disposedin the reservoir 38 for recording the temperature T3 of the water in thereservoir 38. The connecting pipe 40 is opened and closed by thecontrollable connecting valve 42. The valve 42 can be controlled via aline 42 s to the control unit 58. If the valve 42 is closed and thevalve 50 is open, the reservoir 38 is filled with cold fresh water. Ifthe valve settings are reversed, it is filled with water from thecirculation path which can be heated if necessary.

The reservoir 38 is implemented in the form of a container disposedparallel to the sidewall of the tub 14 and abutting said sidewall. Thethird temperature sensor 36 is disposed in contact with the wall of thereservoir 38 facing the tub 14. To improve the heat drying efficiency,the reservoir 38 is filled with cold fresh water during the drying cycle12, which means that the sidewall of the tub 14 facing the reservoir 38becomes a cooled condensation surface. On the one hand, therefore, thetemperature sensor 36 fulfills the same purpose as the sensor 34 in thelast described example. However, as it is only in the fresh water flowof the circulating pump 20, it can capture the output temperature of thefresh water more precisely than the temperature sensor 32. Consequently,it provides a better data set for load determination by the control unit58.

LIST OF REFERENCE CHARACTERS

-   2 pre-wash-   4 heat-up phase/heat up-   6 cleaning cycle/clean-   8 intermediate wash cycle/intermediate wash-   10 rinse-   12 drying cycle/drying-   14 tub-   16 loading door-   18 control panel-   19 base plate-   20 circulating pump-   20 s control line for circulating pump-   22 a feed-   22 b drain-   24 rotary spray arm-   28 wash items-   30 dish rack-   32 first temperature sensor (circulation path)-   34 second temperature sensor condensation surface (e.g. loading    door)-   36 third temperature sensor (heat exchanger)-   38 heat exchanger-   40 connecting pipe-   42 connecting valve-   42 s control line for connecting valve-   44 supply-   48 supply pipe-   52 drain pipe-   56 heater-   56 s control line for heater-   58 control unit

1-11. (canceled)
 12. A method for detecting a load-related change inthermal capacity of a water-bearing domestic appliance for optimizing adrying process, comprising: measuring a temperature trend during coolingof wash items in the appliance.
 13. The method as claimed in claim 12,wherein the temperature trend is a temperature trend of wash liquor in acirculation path of the appliance.
 14. The method as claimed in claim12, wherein the temperature trend is a temperature trend of acondensation surface of the appliance.
 15. The method as claimed inclaim 12, wherein the temperature trend is a temperature trend of awater reservoir of the appliance.
 16. The method as claimed in claim 15,wherein the temperature trend is a temperature trend of wash liquormixed with fresh water measured in at least one of the circulation pathand the condensation surface.
 17. The method as claimed in claim 12,wherein the temperature trend is measured over a predefined period oftime.
 18. The method as claimed in claim 12, wherein the temperaturetrend is measured within a predefined time interval.
 19. The method asclaimed in claim 12, wherein the temperature trend is measured at leastone of continuously and at predefined intervals.
 20. The method asclaimed in claim 12, wherein the temperature trend is a temperaturetrend of wash liquor measured during a wash liquor heat-up phase.
 21. Awater-bearing domestic appliance, comprising: a sensor structured todetect a load-related capacity for storing thermal energy, and tomeasure a temperature trend during cooling of wash items disposed in theappliance.